In many cases of object manipulation by robot, it is required to know the mass and center of mass of the object. For the case when the weight or shape of an object is over the grasp capacity of a robot hand, we propose a technique that can estimate the mass and center of mass of a graspless unknown object, which has curved surfaces and a base plane. A line, called the passing-CM line, which contains the center of mass is defined. For estimating the passing-CM line, a tipping operation by the robot finger which tips the object slowly and repeatedly in a parallel motion with it vertical operation plane, is proposed. Using the fingertip position and force information measured from the tipping operation, an algorithm to estimate the passing-CM line is described. Then the algorithm to estimate the mass and center of mass of the object is given by estimating the intersect point of several orientation-different passing-CM lines.
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